For today's How-To you'll need:
- Hot air popcorn popper (read on to make sure you get the right
- Two gang plastic electrical box
- Two hole switch cover
- Basic wall
- Light switch
- Two computer power cables, or one fifteen foot extension cord.
- Radio Shack Model: 273-1512 Transformer or similar (25.2V Center tap 2.0A.) [Note: we originally grabbed the
wrong part number]
- Electrical tape or heat shrink tubing
- Soldering iron or twist on
- Green coffee beans
Find the right popper
Not every hot air popper is
suitable for roasting coffee. In order to safely roast coffee, the hot air chamber should look like this:
The hot air enters from the sides,
allowing the beans to heat evenly and rotate. If the air enters directly from the bottom, the concentrated heat can
become a fire hazard. We used a West Bend Poppery II for our roaster. Finding a suitable popper can be a challenge, but
thrift stores often have suitable poppers for a few dollars. eBay is a great source for poppers, and Walgreens has
reportedly been selling a suitable machine.
Normal air poppers won't get quite hot enough to fully roast
coffee; some modifications are necessary . We'll be removing the "safety" features of the popper, so be aware
that overheating the unit is possible (you know we're not going to be held responsible, yadda yadda). So, most poppers
are built with two heating coils. One is used to reduce the voltage powering the fan motor. Sometimes one of the coils
has failed, so test the popper to make sure it gets hot enough to pop popcorn. If the test resulted in a nice snack,
it's probably in full working order.
When the project is done, the popper
will have two separate circuits. The dimmer will be used to control the fan, while the switch will turn the heating
coils on and off. For more detail peep our super-detailed schematic above, that should give you a very precise
Radio shack sells a couple of 25.2 volt center tap transformers. Make sure to get the
2.0 Amp version. If you pick up the 450ma version, the fan on the popper won't go fast enough (and the transformer will
get pretty warm). Get an outdoor style plastic two gang electrical box (some people get a 3 gang and mount the
transformer inside the box).
Mount the transformer on the control box
We drilled two holes and
used a pair of 6-32 nuts and machine screws. There are a couple of wire to wire connections. You can twist these
together and use wiring nuts, or solder them and insulate them with electrical tape or quality heat shrink tubing.
Prepare your power wires
You need four total or two pair of wires from the popper to the control box,
and a pair of wires leading to a power plug. Keep in mind that green is used for a safety ground in AC wiring. If you're
using a cord with a three prong plug on it, the other two will be the 'hot' wires.
One of the hot leads will connect to one pole of the switch and the dimmer. The other will connect to one lead from
the heating coil in the popper and a primary wire to the transformer. The other pole of the switch connects to the
second lead to the popper's heating element. The other wire of the dimmer will connect to the other primary lead of the
transformer. Finally, the two outer leads (yellow in the photo) are connected to the other pair of wires to the popper.
Those two will be connected to the fan motor. Label each pair of wires "fan" and "heat" so you
don't have to trace which is which later on.
Double check all of your wiring for safety!
popper needs to be opened up so that the fan can be separated from the heating element and the new wires from the
control box can be connected. Poppers vary, but the most use a few screws to hold the body together.
Our Poppery II used three phillips
screws. Remove them and the top should easily separate from the base. The wiring will usually keep the heating chamber
from being removed from the body until you remove the power cable strain relief.
ours, the stress relief clip was easily pried up using a regular screw driver. If you can't get a screw driver under it,
try a good pair of pliers. Don't worry about damaging the cable, we'll be replacing it. Once it's out, just pull the two
halves apart and the heating chamber should now be easily removed.
is attached to the bottom of the heating chamber with a few screws. Before we start, note the three wires that connect
to the two heating elements, and the two leads to the motor. Cut off the power cord and cut all the wires near the
Remove the three screws that hold the
fan to the heating chamber. Now the heating chamber can be pulled apart to reveal the heating coils. When the chamber
comes apart, you'll find a metal spacer and a fiber gasket, made of the same heat resistant material that the heating
coils are mounted to. On reassembly, they should line up easily. Just in case, the order is: heating coils, metal
spacer, fiber gasket, fan assembly.
The silver component with the red labeling
is a thermal fuse. The black component with the brass track on it is a thermal switch. The thermal fuse is the
feature of the popper. If the popper overheats, the fuse kicks in and power to the heating coil
The thermostat works by opening the contacts once a particular temperature is reached. Use a
piece of stiff wire (uninsulated if it's not rated for the heat) and connect the terminal of the black wire to the
terminal at the end of the thermal switch, next to where the white wire is terminated. Make sure it won't short across
the other terminals! You have now DISABLED
feature of the popper in order
to gain full manual control. Now the popper can get hot enough to roast the coffee beans. It can also easily get hot
enough to start a fire if you don't pay attention to it. In a nutshell: never run the heating element without the fan
blowing. For safety, use a power strip with a circuit breaker in it.
Pull the two new pairs of power leads
through the hole in the body previously used for the power cord. Connect the fan leads directly to the original fan
wires. For the heater, connect the wires to their respective devices. Now reassemble the popper.
To add some stress relief, we
use zip ties on the control wires inside and outside the popper body. There are two posts on the bottom of the fan base
that fit into two molded towers inside the body. Once you line these up, you can put the top back on the body and screw
the unit back together.
If you're satisfied with your
wiring job, install the switch plate cover on your control box, along with the knob for the dimmer. Once you feel
confident, test the unit out in safe conditions. We suggest placing it on concrete, away from anything flammable just
in case! As long as you have good home wiring, a short will probably just trip a breaker. If you have problems, unplug
it, make sure everything is cooled off and trace your circuits.
the roaster is built and tested, you'll probably want to roast some coffee. One of our favorite coffee suppliers is Sweet Maria's.
Turn the fan dimmer all the way up and turn the heater
on. Add green coffee beans until they just stop spinning around. Controlling the heat level will seem a bit
un-intuitive. The faster the fan is going, the cooler the beans will be. In order to heat the beans up more, you need
to slow down the fan slightly. It will take a batch or two of beans to get used to the process. When the beans have
reached the roast level you want, crank the fan on high and turn off the heater to cool the beans.
Maria's has a short how-to on air popper roasting
great pictorial showing the stages of roasting
. Make sure you check them out. If you're not sure what kind of beans to get, try one of their sample packs.
For testing out your new roaster, they offer a cheap coffee called Ugh! If you just want to try out the whole roasting
experience, try sweet talking a local roaster into a few greens for educational purposes. Happy caffeination!